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Posted: Wed Jul 10, 2013 9:13am

Post Subject: BMC 1.8 Diesel

We have a BMC 1.8 Diesel in our 1980 Aquafiber 35 with a sliding roof. The engine runs fine apart from burning a little oil. I have been trying to check how many litres of fuel it's using per hour and so far it looks like about 4 or 5. Would this be about right for this type of craft? Would it be worth considering changing the engine to a marinized Peugeot 1.9 for better economy?


Posted: Wed Jul 10, 2013 12:35pm

Post Subject: BMC 1.8 Diesel

I think that boat is a heavily built wide beam, possibly ex hire from the Broads or Fens. I also suspect you are using it at sea or on a river and driving it hard. Virtually any engine in a canal boat will return between 1 and 1.5 litres per hour but that is at around 3 mph. As you start to drive the boat faster the water resistance increases at something like the square of the speed, so double your speed and the water resistance (and power required and thus fuel consumption) tends to increase by 4. Not only that but give or take a bit a 35 ft boat (say 33 at the waterline) will have a maximum displacement speed of about 7 knots. If you try to exceed that you might just as well pour your fuel over the side. It will just make larger and larger waves. How much of this rings true? I simply can not understand why anyone would think it a good idea to take out a robust gear/chain timed engine and replace it with one that will return a similar consumption but put your wallet at the mercy of a glorified rubber timing band. Unless you go for a really modern diesel with electronic, common rail injection I can not see any way you are likely to get a significantly better fuel consumption. Even then I doubt the improvement would ever repay the costs and the possible reliability in marine use issues. Tony Brooks

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